The present invention relates to an apparatus for reading an original, and more particularly to an original reading apparatus which can read a color original at high speed.
2. Related Background Art
There is known an original reading method. With this method, the surface of an original is scanned by a line sensor to read it.
FIGS. 1 and 2 are schematic circuit diagrams showing monochromatic line sensors. In the circuit shown in FIG. 1, signals outputted from a photodiode array composed of a plurality of light receiving elements or photodiodes , are inputted to input ports 103a of a CCD analog shift register 103 via switching transistors 102 which are turned on by a transfer signal 101. The signals inputted to the CCD analog shift register 103 are picked up as a video signal from a video signal output terminal 105, in real time in response to a clock signal 104.
In the circuit shown in FIG. 2, signals outputted from a photodiode array 100 are picked up as a video signal from a video signal output terminal 107, in real time in response to a clock signal 104 applied to a digital shift register 106 which turns on switching transistors 106.
Three color signals including red, green and blue can be read by scanning an original with these line sensors. Methods of reading three color signals are: (1) for example, a line sequential method in which a monochromatic line sensor as shown in FIGS. 1 and 2 is used to read a line of an original while the color of light applied to it is changed; (2) a method in which an original is read by providing red, green and blue filters on the light incident side of a line sensor, as shown in FIG. (3), and (3) a method in which an original is read by using three line sensors provided with red, green and blue filters 111, 112, and 113, respectively, as shown in FIG. 4.
With the line sequential method, however, the color of light must be changed for reading each line of an original. For example, when an original is read at a resolution of 400 dpi (dot/inch) and at a scan speed of 60 mm/sec, the light switching time becomes about 0.35 m sec for each dot. Further, it is necessary to use a light source which has no afterglow and can be switched at high speed. For such a light source, a light emitting diode (LED) has been used heretofore. However, a light emitting diode cannot radiate blue color light sufficiently, and therefore a fluorescent lamp has been used for blue color light. Use of a fluorescent lamp necessitates a high speed switching circuit and the like to speed up a response time of the lamp. As such, the line sequential method operated at high speed requires additional circuits, resulting in a high cost of the apparatus.
With the method shown in FIG. 3, the width of a photodiode element becomes about one third of the element pitch. Therefore, in case of a so-called contact type line sensor disposed in contact with or adjacent an original and having a resolution of 400 dpi, the width of a light receiving element of the line sensor becomes smaller than 20 micron. Therefore, an elongated line sensor is difficult to manufacture.
With the method shown in FIG. 4 using three photodiode arrays, the drawback occurs that a picking up the output of the second array is difficult because of the problem of an arrangement of the line sensor.